Project Summary Air-purifying respirators are worn in occupational settings to protect against respiratory hazards, including toxic vapors and gases. Unfortunately, the user generally does not know when to change the cartridge before breakthrough occurs. It is therefore essential to have an end-of-service-life indicator (ESLI) system integrated with the respirator cartridge to warn the user when it is time to change cartridges. Current challenges in developing ESLI systems include the effects of humidity, as well as selectivity, size, weight, and power restrictions. Additionally, manufacturers prefer to limit sensor costs to about $1 per cartridge, and $20?$50 for sensor-related fixtures and electronics. Only a limited number of colorimetric passive ESLIs are commercially available, and they rely on subjective visual identification of a color change. Users must routinely monitor the sensor for color change, which distracts from their work. No active ESLI is NIOSH-approved. Intelligent Optical Systems, Inc. (IOS) proposes to develop a low cost active ESLI system by incorporating sensitive optical fibers into the respiratory cartridge to monitor for contaminants; an audible/visual alarm will be triggered when the cartridge reaches 90% saturation.